The invention relates to polymer oxidates with improved dispersability in aqueous media, their preparation by oxidative degradation of polyethylene plastic, and their use.
Ethylene homo- and copolymers which have the nature of plastics may be reacted to give polar products via controlled oxidative degradation. Polymer oxidates of this type have oxygen-containing functional groups, e.g. carboxyl, ester and ketone groups, and, due to the polarity which these give, are suitable for preparing aqueous dispersions for applications in polishes or the industrial sector for example. Polyethylene oxidates also serve as auxiliaries for processing and shaping plastics (cf. Ullmann""s Encyclopaedia of Industrial Chemistry, 5th Edn, Vol. A28, pp.155-156, Weinheim 1996).
Processes for the oxidative degradation of polyethylene plastic are known. For example, this may be done by treating the finely divided raw material at below its softening point with oxygen or with oxygen-containing gases (DE-A-1 520 008). A variant of this procedure is to carry out the reaction in the presence of an inert dispersion medium, e.g. water (DE-A-3 238 652). The oxidative degradation of ethylene homo- and copolymers at temperatures above the softening point in the presence of an inert dispersion medium has also been described (DE-A-2 035 706, EP-A-28 384). The polyethylene used for the oxidation is prepared by ethylene polymerization with the aid of titanium and/or chromium transition metal catalysts by the low-pressure process or by the free-radical high-pressure process. It is also known that low-molar-mass polyethylene waxes can be oxidized to give wax oxidates. These are obtained by bringing oxygen-containing gases into contact with the melts of the raw materials. The starting material here has low viscosity and it is therefore unnecessary to add a dispersion medium. The oxidation of polyethylene waxes whose weight-average molar Mass is below 40,000 g/mol, generally below 10,000 g/mol, gives products whose type differs from those obtained from high-molar-mass polyethylene which has the nature of plastic. Wax oxidates have relatively low molar mass and are softer than polymer oxidates, and if used in floor covering polish formulations, for example, they are inferior to polymer oxidates in terms of hardness and resistance to foot traffic. Wax oxidates are disclosed, for example, in DE-A-1 227 654.
Aqueous dispersions prepared using the abovementioned polymer oxidates frequently have disadvantages in practical use, for example insufficient transparency, excessive viscosity, reduced gloss after film application or unsatisfactory storage stability.
The object was therefore to provide polymer oxidates with improved dispersion and usage properties and not having the disadvantages of previously known polymer oxidates.
Surprisingly, it has now been found that polymer oxidates with improved properties are obtained if the high-molar-mass polymer used as starting material has been prepared with the aid of metallocene catalysts.
The invention therefore provides polymer oxidates wherein a starting polymer, which is a polyethylene obtained using metallocene catalysts and having a weight-average molar mass above 40,000 g/mol, is oxidized by reaction with oxygen or oxygen-containing gas mixtures.
Starting Polymer
Polymerization processes for preparing polyethylene with the aid of metallocene catalyst systems are known. A suspension process is described in EP-A-578 838, there are details for the gas-phase process in EP-A-323 716, for example, and the high-pressure process in EP-A-361 866, for example.
Suitable starting polymers are polyethylenes prepared using metallocene catalysts, and for the purposes of the present invention these are ethylene homo-, co- or terpolymers with weight-average molar masses Mw above 40,000 g/mol, preferably from 50,000 to 4,000,000 g/mol. The melt viscosities of polymers of this type, measured at 140xc2x0 C., are above 50,000 mPa.s, usually above 100 Pa.s. The melt flow rate MFR 190/5, which is the variable usually used for polymer viscosities, is below 100 g/10 min, usually less than or equal to 50 g/10 min.
Besides ethylene, this starting polymer may also comprise amounts of from 0.1 to 20% by weight, based on total polymer, of (xcex1-olefins with chain lengths from 3 to 30, preferably from 3 to 8, e.g. propene, 1-butene, 1-hexene or 1-octene. Particular preference is given to ethylene copolymers with up to 10% by weight of propene or 1 -butene.
Preferred starting polymers have narrow molar mass distribution Mw/Mn of below 5, preferably below 4, in particular less than or equal to 3.5.
Preparation of the Starting Polymer
Catalyst systems for preparing the starting polymer are composed of one or more metallocene compounds (I) together with a cocatalyst (II), and also, if desired, another catalyst component (III) and a scavenger (IV). The catalyst system is frequently used after fixing on a support, in which case the species active for polymerization is produced either prior to, during or after application to the support, depending on variations in the order in which components (I) to (IV) and the support are used.
The metallocene compound (I) typically has the formula I 
and includes, for example, structures of the formulae (Ia): 
of the formula Ib: 
or of the formula Ic: 
M1 in the formulae is a metal of group IVb, Vb, VIb or VIIIb of the Periodic Table, for example titanium, zirconium, iron, chromium, cobalt or nickel.
R1 and R2 are identical or different and are a hydrogen atom, a C1-C10-alkyl group, a C1-C10-alkoxy group, a C6-C10-aryl group, a C6-C10-aryloxy group, a C2-C10-alkenyl group, a C7-C40-arylalkyl group, a C7-C40-alkylaryl group, a C8-C40-arylalkenyl group or a halogen atom.
R3 and R4 are identical or different and in the embodiments of the formulae (Ia) to (Ic) are alkyl, pyridyl, arylalkyl, alkylaryl, or mono- or bisalkylpyridyl-substituted imido radicals, which may have bridging to one another via alkyl, bisalkylaryl or bisalkylpyridyl groups. One of the radicals R3 and R4 may moreover be a substituted nitrogen atom, in which case R28 is defined as for R21 and is preferably methyl, tert-butyl or cyclohexyl.
R5 to R14 are identical or different and, independently of one another, are a hydrogen atom, a halogen atom, a C1-C18-aryl group, a C1-C10-alkoxy group, or an xe2x80x94NR202xe2x80x94, xe2x80x94SR20xe2x80x94, xe2x80x94OSiR203xe2x80x94, xe2x80x94SiR203xe2x80x94, or xe2x80x94PR202xe2x80x94 radical, where R20 is a C1-C10-alkyl group or a C6-C10-aryl group, or, in the case of radicals containing Si or P, also a halogen atom, or two adjacent radicals R5, R6, R7, R8, R9, R10, R11, R12, R13 or R14, together with the carbon atoms connecting them, may form a ring system. This gives, for example, 2-methyl-4-naphthylindenyl or substituted benzoindenyl or fluorenyl. Preferred substituents for these ligands have been given in the description of R1.
R17 is: 
xe2x95x90BR21, xe2x95x90AIR21, xe2x80x94Gexe2x80x94, xe2x80x94Snxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x95x90SO, xe2x95x90SO2, xe2x95x90NR19, xe2x95x90CO, xe2x95x90PR19 or xe2x95x90P(O)R19, where R21, R22 and R23 are identical or different and are a hydrogen atom, a halogen atom, a C1-C30-alkyl group, a C1-C10-fluoroalkyl group, a C6-C10-fluoroaryl group, a C6-C10-aryl group, a C1-C10-alkoxy group, a C2-C10-alkenyl group, a C7-C40-arylalkyl group, a C8-C40-arylalkenyl group or a C7-C40-alkylaryl group, or R21 and R22 or R21 and R23, in each case together with the atoms connecting them, may form a ring having from 2 to 8 ring-carbon atoms.
M2 is silicon, germanium or tin, preferably silicon. R17 is preferably xe2x95x90CR21R22, xe2x95x90SiR21R22, xe2x95x90GeR21R22, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x95x90SO, xe2x95x90PR21 or xe2x95x90P(O)R21.
R15 and R16 are identical or different and are as defined for R21.
m and n are identical or different and are 0, 1 or 2.
R18 and R19 are as defined for R21 and R22.
Illustrative examples of metallocenes are:
bis(1-n-butyl-3-methyl-cyclopentadienyl)zirconium dichloride,
bistetrahydroindenyl(dimethyl)zirconium,
dimethylsilyl-bis-1-(2-methyl-4,5-benzoindenyl)(dibenzyl)zirconium,
dimethylsilyl-bis-1-(2-methyltetrahydroindenyl)zirconium dichloride,
ethylenebis-1-(2-methyl-4-phenylindenyl)zirconium dichloride,
ethylenebis-1-(tetrahydroindenyl)zirconium dichloride,
isopropylidene(1-indenyl)(cyclopentadienyl)zirconium dichloride,
isopropylidene(9-fluorenyl)(cyclopentadienyl)zirconium dichloride,
(tert-butylamido)dimethyl(tetramethylcyclopentadienyl)silyl(dimethyl)-titanium,
2,6-bis[1-(2-tert-butylphenylimino)ethyl]pyridineiron dichloride, and
2,6-bis[(2,6-di-isopropylphenylimino)methyl]pyridinecobalt dichloride.
Suitable cocatalysts (II) are organoaluminum compounds, in particular aluminoxanes, or else aluminium-free systems, such as R26xNH4xe2x88x92xBR274, R26xPH4xe2x88x92xBR274, R263CBR274 or BR273. In these formulae x is a number from 1 to 4, and the radicals R26 are identical or different and are C1-C20-alkyl or C6-C12-aryl, or two radicals R26, together with the atom connecting them, may form a ring having from 2 to 8 ring-carbon atoms. The radicals R27 are identical or different and are aryl, which may have alkyl-, haloalky- or halo-substitution. Examples of R26 are ethyl and phenyl, and an example of R27 is pentafluorophenyl. Aluminoxanes, prepared by controlled hydrolysis of tialkylaluminum compounds, or mixtures of these, in particular methylaluminoxane, are generally successful for activation.
Component (III) is a cyclic boroxine of the formula (R21BO)3, such as trimethylboroxine, where the radicals R21 may also differ from one another. Other compounds of type (II) may also be used instead of this to increase the activity of the catalyst system.
In addition, use may be made of the scavenger (IV) to maintain protection of the polymerization system from polar catalyst poisons. Any organoaluminum compound, such as triethylaluminum or triisobutylaluminum, or also mixtures of these, is suitable for this purpose.
Oxidation of the Starting Polymer
The oxidation is carried out in such a way as to give the polymer oxidates obtained acid numbers of from 0.5 to 200 mg KOH/g and melt viscosities at 140xc2x0 C. of from 50 to 20,000 mPa.s, preferably from 100 to 20,000 mPa.s.
The oxidative degradation of the high-molar-mass starting polymer is carried out by exposure to oxygen or oxygen-containing gases, preferably air, at at least atmospheric pressure. The temperature used here may be below or above the softening point of the polymer. In the latter case the presence of an inert dispersion medium, such as water, is generally necessary to ensure that the phase boundary area between the polymer melt and the oxidizing agent is sufficiently large to give rapid reaction. If the oxidation is carried out at below the softening point, the reaction proceeds at a lower rate, but the addition and subsequent removal of dispersion medium can then be dispensed with. The progress of the reaction can be followed using the acid number. Small amounts of previously oxidized polyethylene can be added as a xe2x80x9cstarting aidxe2x80x9d for the reaction.
The oxidative degradation to give polymer oxidates may be additionally promoted by pretreatment of the polymer with peroxide or azo compounds, such as 2,2xe2x80x2-azobis(2-methylpropiononitrile) or by oxidative plasma treatment. When selecting the raw materials preference should be given to unstabilized polymers or those with a low proportion of antioxidants.
Controlled addition of acids, bases or buffer substances can shorten the reaction time and prevent crosslinking reactions. The polymer oxidation may be carried out continuously or batchwise.
Use of the Polymer Oxidates
The present invention further provides the use of the novel polymer oxidate for preparing aqueous dispersions, as a material added when dispersing pigments, e.g. in masterbatches, or for surface-modification or polar modification of polyolefins. Another advantageous application sector is use as a lubricant for plastics processing.
The polymer oxidates may be processed to give aqueous dispersions using auxiliaries, such as ionic or nonionic emulsifiers, bases, such as alkali metal hydroxides or ammonia or amines, and other additives. Among the quality criteria for dispersions of this type are viscosity, storage stability at elevated temperatures, transparency, and also the gloss of the films which form when the dispersions are applied to smooth surfaces.
The functional groups in the polymer oxidate may also be derivatized by reactions at the carboxyl, ester and keto groups. This gives additives advantageously used for surface-modification or pigment dispersion, or compatibilizers for polymer blends.